Recently, hybrid vehicles and electric vehicles are increasingly noted as environmentally conscious vehicles, and some hybrid vehicles are commercially available.
The hybrid vehicle is a vehicle having as a power source a conventional engine and in addition thereto a DC power supply, an inverter, and a motor driven by the inverter. In other words, the engine is driven to obtain a power source and furthermore the DC power supply provides a DC voltage which is in turn converted by the inverter to an AC voltage which is used to rotate the motor to obtain a power source.
Electric vehicles are vehicles having as a power source a DC power supply, an inverter and the motor driven by the inverter.
For such a hybrid or electric vehicle it has also been considered to up-convert a DC voltage received from a DC power supply by an up converter and supply the up-converted DC voltage to an inverter driving a motor (Japanese Patent Laying-Open No. 8-214592).
More specifically, the hybrid or electric vehicle has a motor drive apparatus, as shown in FIG. 42, mounted therein. With reference to FIG. 42, a motor drive apparatus 400 includes a DC power supply B, system relays SR1, SR2, capacitors C1, C2, a bidirectional converter 310, a voltage sensor 320, and an inverter 330.
DC power supply B outputs a DC voltage. System relays SR1, SR2 are turned on by a control device (not shown) to supply the DC voltage received from DC power supply B to capacitor C1. Capacitor C1 smoothes the supplied DC voltage and supplies the smoothed DC voltage to bidirectional converter 310.
Bidirectional converter 310 includes a reactor L1, NPN transistors Tr1, Tr2, and diodes D1, D2. Reactor L1 has one end connected to a power supply line of DC power supply B and the other end to a point intermediate between NPN transistors Tr1 and Tr2, i.e., between the emitter of NPN transistor Tr1 and the collector of NPN transistor Tr2. NPN transistors Tr1 and Tr2 are connected in series between the power supply line and an earth line. NPN transistor Tr1 has a collector connected to the power supply line and NPN transistor Tr2 has an emitter connected to the earth line. Furthermore between the collectors and emitters of NPN transistors Tr1 and Tr2, respectively, diodes D1 and D2, respectively, passing a current from their corresponding emitters to collectors, respectively, are arranged.
Bidirectional converter 310 is controlled by a control circuit (not shown) to turn on/off NPN transistors Tr1, Tr2 to up convert the DC voltage supplied from capacitor C1 and supply an output voltage to capacitor C2. Furthermore, when the hybrid or electric vehicle with motor drive apparatus 400 mounted therein is regeneratively braked, bidirectional converter 310 down-converts a DC voltage generated by an AC motor M1 and converted by inverter 330 and supplies the voltage to DC power supply B.
Capacitor C2 smoothes the DC voltage supplied from bidirectional converter 310 and supplies the smoothed DC voltage to inverter 330. Voltage sensor 320 detects a voltage across capacitor C2 i.e., a voltage Vm output from bidirectional converter 310.
When inverter 330 receives DC voltage from capacitor C2, inverter 330 is controlled by a control device (not shown) to convert the DC voltage to an AC voltage to drive AC motor M1. Thus AC motor M1 is driven to generate a torque designated by a torque command value.
Furthermore when the hybrid or electric vehicle with motor drive apparatus 400 mounted therein is regeneratively braked, inverter 330 is controlled by the control device to convert an AC voltage generated by AC motor M1 to a DC voltage and supply the converted DC voltage via capacitor C2 to bidirectional converter 310.
Thus in motor drive apparatus 400 when AC motor M1 is driven DC power supply B provides a DC voltage which is in turn up-converted and supplied to inverter 330 and when AC motor M1 is regenerated AC motor M1 generates a DC voltage which is in turn converted by inverter 330, and down-converted and supplied to DC power supply B.
Conventional motor drive apparatuses, however, determines from an AC motor's load in magnitude whether to permit or prohibit up conversion control and down conversion control. As such, up conversion control or down conversion control is also effected for a range for which a reactor current inverts in polarity, resulting in disadvantageously increased switching noise and switching loss attributed to NPN transistors Tr1 and Tr2.
With reference to FIGS. 43 and 44 a conventional disadvantage will now be more specifically described. FIG. 43 is timing plots of a reactor current IL uninverted in polarity, currents ITr1 and Itr2 passing through NPN transistors Tr1 and Tr2, and currents ID1 and ID2 passing through diodes D1 and D2. FIG. 44 are timing plots of reactor current IL inverted in polarity, and currents ITr1 and ITr2 passing through NPN transistors Tr1 and Tr2, and currents ID1 and ID2 passing through diodes D1 and D2.
With reference to FIG. 43 shows a case in which the reactor current uninverted in polarity is a positive reactor current IL, i.e., bidirectional converter 310 performs an up-converting operation. A period from time t1 to time t2 corresponds to one cycle of controlling NPN transistors Tr1 and Tr2 in the up-converting operation.
From time t1 through time t3 NPN transistor Tr2 is turned on and a DC current flows through a circuit formed of DC power supply B, reactor L1 and NPN transistor Tr2 from DC power supply B toward NPN transistor Tr2 (hereinafter this direction will be referred to as a positive direction) and reactor L1 stores power. In other words, during this period, current ITr2 flowing through NPN transistor Tr2 increases and so does reactor current IL. At time t3 NPN transistor Tr2 is turned off and NPN transistor Tr1 is turned on. In response, current ITr2 decreases to 0 A and from time t3 through time t2 a DC current flows from reactor L1 via diode D1 toward capacitor C2 in accordance with the power stored in reactor L1.
In that case, current ID1 flowing through diode D1 gradually decreases as time t2 is approached. Accordingly, reactor current IL also decreases as time t2 is approached.
Consequently for the one cycle from time t1 to time t2 NPN transistor Tr1 and diode D2 do not conduct and currents ITr1 and ITr2 are 0 A. Furthermore in this one cycle NPN transistors Tr1, Tr2 are switched only at time t3.
Such an operation is repeated and bidirectional converter 310 performs the up-converting operation, and power supply current Ib output from DC power supply B will be a current corresponding reactor current IL averaged.
With reference to FIG. 44, for the reactor current inverted in polarity, from time t1 through time t4 NPN transistor Tr2 is turned on and at time t4 NPN transistor Tr2 is turned off and NPN transistor Tr1 is turned on. Accordingly from time t1 through time t4 a DC current flows in the positive direction through a circuit formed of DC power supply B, reactor L1 and NPN transistorTr2 and reactor L1 stores power. More specifically in this period current ITr2 flowing through NPN transistor Tr2 increases and reactor current IL also increases. At time t4 NPN transistor Tr2 is turned off and NPN transistor Tr1 is turned on, and current ITr2 decreases to 0 A, and from time t4 through time t5 a DC current flows from reactor L1 via diode D1 toward capacitor C2 in accordance with power stored in reactor L1.
In that case, current ID1 flowing through diode D1 gradually decreases as time t5 is approached. Accordingly, reactor current IL also decreases as time t5 is approached.
At time t5, reactor current IL switches in polarity from positive to negative. In other words, bidirectional converter 310 performs a down-converting operation. Accordingly, from time t5 through time t6 a DC current flows in a direction from capacitor C2 via NPN transistor Tr1 to DC power supply B, and during this period, current ITr1 flowing through NPN transistor Tr1 increases in a negative direction and reactor current IL flowing in the negative direction increases.
Subsequently at time t6 NPN transistor Tr1 is turned off and NPN transistor Tr2 is turned on. In response, current ITr1 decreases to 0 A (indicating that the current flowing in the negative direction decreases) and a circuit formed of DC power supply B, diode D2 and reactor L1 passes a DC current in the negative direction, and current ID2 flowing through diode D2 decreases as time t2 is approached, and reactor current IL also decreases (indicating that a current flowing in a direction from NPN transistor Tr2 toward DC power supply B decreases).
Consequently for the one cycle from time t1 through time t2 NPN transistors Tr1, Tr2 are switched at time t4 and time t6.
Such an operation is repeated and bidirectional converter 310 performs up-converting and down-converting operations. DC power supply B receives/outputs power supply current Ib, which is a current corresponding to reactor current IL averaged and in this case it is 0 A.
As has been described above, NPN transistors Tr1, Tr2 with reactor current IL uninverted in polarity are switched only once during the 1-cycle control period and with reactor current IL inverted in polarity are switched twice in the period.
In other words, a range for which the motor's load reduces and the reactor current's polarity is inverted is also accompanied by an increased frequency of switching of the NPN transistors configuring the bidirectional converter if typical up-converting and down-converting operations are performed. As they switch more frequently, the NPN transistors generate noise more frequently and thus increasingly. Furthermore, as they switch more frequently, the transistors also provide increased switching loss.